The goal of this work is to understand at the molecular level the mechanisms of mother cell specific transcriptional regulation in Saccharomyces cerevisiae. The yeast HO gene is transcribe only in mother cells, one of the two cell types descending from each mitotic division. An analogy can be drawn between this lineage specificity in HO expression and the differential gene regulation which occurs during embryonic development. Two genes, SWI5 and SIN3, have been identified as determining the mother cell specificity of HO expression. SWI5 and SIN3 are positive and negative, respectively, transcriptional regulators. SWI5 encodes a phosphorylated transcriptional activator which binds to the HO promoter. Another DNA-binding protein called "R", a possible repressor, has been identified binding adjacent to the SW15 binding site. This DNA-binding activity is absent in extracts prepared from sin3- mutants. We plan to analyze the interactions of these two proteins with DNA and with each other to understand the mechanisms underlying differential HO expression in mothers and daughters. We have demonstrated that SIN3 does not encode R. Our data indicates that R is absent in extracts prepared from sin3- mutants because it is bound by an inhibitor protein. We believe that SIN3 regulates the in vitro DNA-binding activity of R by sequestering the inhibitor. This mechanism of regulation of DNA-binding activity by protein-protein interactions may prove to be common, and we plan to determine how SIN3 regulates DNA-binding proteins. An additional objective is to determine how SWI5 and SIN3 fit into the global regulatory network of the cell. We have identified other yeast promoters regulated by SW15 and have obtained evidence that SIN3 acts as a regulator of sugar metabolism.